Recognition of the advantages obtained by the targeting and/or the controlled release delivery of therapeutic and diagnostic agents has inspired a lot of research and development of a variety of carrier systems. These range from a general purpose controlled or sustained release systems to systems which are specifically designed to suit a particular application. Depending on the type and nature of the active substance involved, numerous systems for the delivery of antibiotics, vitamins, proteins, etc. have been developed. A number of different carrier materials, from alginate or agar beads and phospholipid coatings or liposomes to very sophisticated polymeric materials, are known or currently in use for the encapsulation of active substances. However, many of the known systems are either too specific, i.e. dedicated to a single substance or at most to a single class of substances, and therefore are of little help when different active substances are considered. Being specifically chosen to carry a specific substance, many of the known delivery vehicles do not provide sufficient flexibility in modifying their release characteristics or biodegradability. Any changes of the nature of the carrier and/or the active to inactive ingredient ratio inevitably requires additional experimentation.
Furthermore, systems known so far do not lend themselves to the production of floating microparticles or floating tablets which can carry different active ingredients. They neither provide for convenient coupling of different functions or incorporation of different active substances within the same microcapsule, such as for example incorporation of a therapeutically active ingredient in the outer encapsulating membrane and a diagnostically active ingredient in the core; nor do they provide of-the-shelf biodegradable microcapsules which could be filled at convenience with a suitable medication in a suitable amount.
EP-A-0 458 745 (Sintetica) discloses air- or gas-filled microballoons bound by an interfacially deposited biodegradable membrane. These microballoons are usable as very efficient sound reflectors in the echographic imaging of body cavities and the blood stream. For preparing the microballoons, a filmogenic polymer is dissolved in a mixture of volatile organic solvents and the resulting organic solution mixed with an aqueous carrier phase to produce an oil-in-water emulsion. The emulsion is then treated, for instance by evaporation or insolubilization, such that the polymer precipitates and deposits to form a membrane at the droplet water/solution boundary. The organic solvent in the microballoons is eventually evacuated and, by lyophilising the suspension, the solvent in the microballoons is replaced by air or a gas. In order to increase their hydrophobicity the microballoons made from biodegradable polymers may contain up to 20% of fats, waxes and high molecular weight hydrocarbons.
U.S. Pat. No. 4,684,479 (D'Arrigo) discloses stabilised bubble suspensions, useful for ultrasound echographic measurements for different applications including echocardiography. The suspensions are formed by vigorously shaking in the presence of air (foaming) mixtures of surfactants with water or mineral oil. The mixtures of surfactants include (a) fatty acid monoglycerides, (b) esters of aromatic acids (like benzoic, phenylacetic, phthalic, salicylic acids, etc.) with sterols (like cholesterol, ergosterol, lanosterol, phytosterol, etc.), (c) a component from the group consisting of sterols, terpenes, bile acids and alkali metal salts of bile acids; and, optionally, (d) sterol esters of aliphatic acids, and (e) a member of the group consisting of glycerol and di- and triglycerides (e.g. dilaurin, trilaurin, dipalmitin, tripalmitin, distearin, tristearin, dimyristin, trimyristin, and the like).
WO-A-93/02712 (Danbiosyst) discloses solid microspheres or hollow (gas or vapour filled) amylodextrin microcapsules prepared by forming a shell from a water soluble starch derivative around a solid or liquid core and subsequently removing the core. The core may be a volatile oil such as perfluorohexane. The microcapsules may be made by an oil-water-oil double emulsion followed by chemical or heat hardening. The microcapsules can be used for echocardiography.